Patent Literature (hereinafter, referred to as “PTL”) 1 discloses a jib connection structure composed of a jib connection shaft projecting in both lateral directions of a boom leading end and jib base end engagement portions provided at a bifurcated jib base end. The jib base end engagement portions are U-shaped and can be engaged with the jib connection shaft. The jib base end engagement portions can be connected to the jib connection shaft by engaging the jib base end engagement portions with the jib connection shaft and inserting pins into through holes in leading ends of the jib base end engagement portions.
For a jib projection operation, a boom is firstly slightly extended, the jib is then moved to a lower holding position of the boom, and the boom is fully retracted. The jib connection shaft is thereby fitted into the jib base end engagement portions. Subsequently, pins are inserted into through holes in the jib base end engagement portions to connect the jib base end engagement portions to the jib connection shaft. Subsequently, the boom is elevated up, so that the jib is suspended from the boom leading end. Lastly, the jib is projected, by exerting tension on a tension rod.
In the meantime, a wire telescoping mechanism is known as a telescoping mechanism of a boom (see, for example, PTL 2). When the wire telescoping mechanism is adopted, simultaneous extension or retraction of a multi-section boom is possible by means of telescopic motion of one hydraulic cylinder. In the wire telescoping mechanism, when for some reasons the balance of the tension between each wire for extension and retraction of the boom is disturbed, the so-called natural extension may sometimes occur in which the boom is not fully retracted despite that the hydraulic cylinder is fully retracted, or in which the boom unexpectedly extends after having been retracted. When the boom naturally extends, there is a problem that a pin interferes with a jib connection shaft and cannot be inserted, so that jib base end engagement portions cannot be connected to the jib connection shaft.
Against this problem, it may be considered to allow clearance between the pin and the jib connection shaft in anticipation of natural extension of the boom. However, an operation in a state where a jib is projected may sometimes cause force acting in a lateral direction on the jib. In this case, there is a problem that the jib swings sideways when the clearance between the pin and the jib connection shaft is too large.